Defense against missile attack has been important for centuries. In the distant past, fortified structures were used to protect against missiles such as projectiles from ballistas, arbalests, and trebuchets. With the advent of projectile-firing cannon, fortified structures became less useful, and the inadequacy of fortifications was exacerbated by the introduction of bomb-carrying aircraft.
More recently, rocket-propelled missiles have become very important, because of their ability to quickly transport extremely destructive payloads to distant locations. The payloads that are now of importance include nuclear, chemical, and biological weapons, known generally as weapons of mass destruction (WMD). These payloads when carried by rocket-propelled missiles are potentially so destructive that a great deal of attention has been directed toward attempts to neutralize the threat. These steps included socio-political solutions such as mutually-assured destruction (MAD). However, the dissolution of a major player, namely the Union of Soviet Socialist Republics (USSR) has reduced the potential efficacy of MAD and allowed the potential or actual proliferation of weapons of mass destruction to small or unstable states and other entities which are not necessarily friendly to the United States.
As a response to the perceived threat to the Unites States of ballistic missiles launched from distant locations and carrying WMD, programs have been instituted to investigate and produce ballistic missile defense systems. Ballistic missiles have an extremely limited time between launch and impact, so defense systems must very quickly identify and destroy the threat.
The ballistic missile goes through several distinct phases during its operation. The first phase is launch, in which a rocket engine lifts the missile and propels it upward. The missile is very vulnerable at this stage, but there are substantial difficulties in identifying it at this stage, as the launch is liable to be in a hostile territory. While the launch may produce a heat (infrared) and light signature that would be identifiable if viewable, there may not be a line-of-sight between sensors and the missile launch that might identify the situation. Spacecraft may be able to view the region, but the communications between the spacecraft and defense systems have not in the past given long warning times of missile launch.
Following launch, the rocket-propelled missile passes through a boost stage, in which the rocket engine propels the missile through a principal portion of the atmosphere. This phase also produces a heat signature. Since the missile is at a significant altitude in this phase, it may be observable by ground-based infrared sensors. The missile may also be observable on ground-based radar systems. Thus, a missile may be identifiable when in the boost phase. At some time, the rocket engine stops operating, so boost thrust goes to zero. Following the termination of thrust, the missile enters a mid-course phase, in which the missile proceeds along a ballistic trajectory, carried by its own inertia.
The missile in its ballistic mode proceeds toward its target. In the mid-course phase, the heat signature is much reduced, but the missile may be clearly viewed by radar. At some point, as the missile approaches its target, it begins to re-enter dense portions of the atmosphere, at which time a further heat signature may be radiated. This re-entry may be at a location essentially above the target. Destruction of the missile during the re-entry phase may still result in damage to the target, since the payload weapon may still be effective and active. Despite the missile being damaged and kept from properly functioning, the constituent parts may still be very harmful to the target region. It is very desirable to identify and destroy missiles very early in flight. There are several reasons for this, 1) to allow time for repeated tries at destruction, and 2) so that the destroyed missile falls short of its target, preferably in the hostile territory.
The destruction of a missile in flight requires the ability to predict the future location of the missile, so that a kill vehicle or laser beam countermeasure can be guided toward the actual location of the target at the time of the arrival of the countermeasure. In the past, kinematic boost phase target identification relied on trajectory template matching techniques. Development of the trajectory templates required the development of large databases of target specific templates relating to the target's temporal kinematic properties such as altitude, velocity, and flight path angle. The efficacy of the trajectory template matching technique depended on having an accurate estimate of time after lift-off (TALO) so that a good estimate of the time index into the templates could be established. The efficacy also depended upon the target having a specific energy trajectory, such as “minimum energy,” “lofted,” or “depressed.” A minimum energy trajectory puts the missile on the target with expenditure of minimum propellant energy. Selection of elevated or depressed launch angles can result in lofted or depressed trajectories. The missile's trajectory should be readily identifiable, so that the temporal templates relevant to one target do not conflict or “smear” with the templates of other targets. When the target deviates from the expected trajectory and/or is lofted or depressed, or when the target time after lift-off (TALO) is not accurately known, the trajectory template methods tend to break down, and do not always produce reliable results.
Improved or alternative target future location estimation methods and/or apparatus are desired.